subsection2-8plus-4minus-46plus4minus4Ablation Study
subsubsection3-4plus-4minus-4-1.5em plus-0.22em minus-0.1emModality Loss 
In this section, we compare different variants of modality loss.
In Table REF , ShowTell indicates the performance of the base model without modality transition. All other models in the table are implemented with MTM.
For comparison, we choose different measurements between the predicted and the target textual encodings, the loss functions include mean absolute error (MAE), mean squared error (MSE), Cosine distance (COS), Kullback-Leibler divergence (KLD), Maximum Mean Discrepancy (MMD) [1]}. From the comparison result, we can see that the standard MSE effectively measures the loss between the preliminary textual vectors and the well-trained global caption representations giving the best performance. However, the measurements such as KLD and COS result in poor performance.
The KLD measures the relative entropy between two distributions, but not directly computing the distances between the input and target vectors. While the COS measures the angles between the two vectors, but the embedding vectors are sensible to magnitude.
The experiments show that they are not suitable for the modality similarity measures.
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